1. Field of the Invention
The invention is in the field of production of high purity hafnium oxide as carried out by the treatment of a substantially zirconium-free, hafnium-loaded, organic solvent derived by solvent extraction techniques attributable to Fischer et al. as described in Z. Anorg-chemistry 255, pages 79-100 and 277-286 (1947), which is referred to in U.S. Pat. Nos. 3,006,719 and 3,069,232 along with an improvement therein by which methyl isobutyl ketone (MIBK) is used instead of ether as an extractant for the hafnium values.
2. Description of the Prior Art
In the derivation and treatment of a substantially zirconium-free, hafnium-loaded, organic solvent by the improved Fischer et al. process for the recovery of high purity hafnium oxide it is usual practice to strip the hafnium-loaded MIBK organic solvent with a sulfuric acid solution and to treat the resulting extract phase with ammonia to precipitate hafnium hydroxide. The precipitate is then calcined to produce the desired high purity hafnium oxide.
This process was allegedly further improved by stripping the loaded solvent with a saturated aqueous solution of ammonium sulfate containing some sulfuric acid, the extract phase containing thiocyanic acid, ammonium sulfate, and the hafnium values in the form of hafnium thiocyanate. Such extract phase is then treated with gaseous ammonia, which reacts with the thiocyanic acid and hafnium thiocyanate to simultaneously form ammonium thiocyanate and hafnium hydroxide. This is said to be of the utmost importance in coprecipitating ammonium sulfate with the hafnium hydroxide to make the hafnium hydroxide easily filterable, see the aforementioned Greenberg et al., U.S. Pat. No. 3,069,232 of Dec. 18, 1962 entitled "Recovery of Hafnium Values." However, not only is the sulfate ion present, but the sulfuric acid introduced into the ammonium sulfate extractant to ensure recovery of the major portion of the thiocyanic acid in the extract phase, contaminates the extract phase and requires additional purification steps.